Interpretive Summary: Stable flies are economically important pests of animals. They are blood- feeders with a very painful bite. Historically, they have had the most impact on cattle in feedlots. Recently they have been suspected to be impacting cattle in pastures. We evaluated the effect of stable flies on the weight gains of yearling cattle in pastures for three years and, for two of those three years, checked if the cattle compensated for weight loses when they were placed in feedlots without stable flies. We found that stable flies had an important economic effect, through reduced weight gains, on cattle in pastures; the effect was very similar to that seen in the traditional feedlot situation. Also, the cattle did not compensate for the reduced weight gains when put in feedlots on high nutrition diets and without stable flies. This is the first controlled study of this nature; it shows that more work is needed in defining why there are stable flies in the pasture and how they can be controlled.

Technical Abstract:
The impact of stable flies, Stomoxys calcitrans (L.) on average daily gains of grazing yearling steers was seen as an average of 0.2 Kg per calf less gain in this three-year study. Cattle were grazing on canyon range pastures in West Central Nebraska. Stable fly numbers over the 3 yr averaged 0.85 per leg on treated calves and 3.64 per leg on control calves. In two of the three years of the grazing trials, the calves were placed in the feedlot and fed a finishing ration. Compensatory gain did not occur in the feedlot after the stable fly stress was removed. Stable flies have a significant economic impact on range cattle and warrant more studies.